Head-up displays were originally developed, during the past 30 years, for displaying cockpit instruments to pilots, first in military, and now in both military and commercial aircraft. HUD technology has been more recently applied to additional applications. Various types of head-up display (“HUD”) systems have been produced, each designed to a unique set of requirements and each possessing certain advantages. HUDs are utilized, for example, to display vehicle sensory and navigation information. A HUD eliminates the need for a vehicle operator to divert his or her eyes from a field of vision, such as from the road, in an automobile application, in order to view important information that may include a vehicle speed relative to various different environmental objects or media, a vehicle orientation relative to various different environmental objects or media, a compass heading, engine performance information, various temperatures, and information provided by other types of sensors and instruments. HUD implementations generally involve placement of optical, mechanical, and electrical components directly between the operator and the operator's necessary field of vision. Generally, these components must meet a set of functional and performance requirements, specific to particular applications, including requirements related to display-light-source brightness, contrast ratio, image quality, minimal obstruction of a viewer's field of vision, minimal attenuation and discoloration of light incoming from a field of vision, suppression of multiple reflections commonly referred to as “ghost images,” “ghosts,” or “ghosting,” and acceptable aesthetic appearance.
Ambient lighting during daytime viewing demands a minimum brightness of 1000 foot-Lamberts from a typical display. This requirement is achieved in many HUDs by choosing bright illumination sources or designing efficient combiner or relay optics. The more efficient the optics, the more they tend to intrude on a viewer's field of vision; conversely, less efficient optics impose greater demands on the illumination source and drive up systems costs. For example, many existing HUDs use vacuum-fluorescent displays (“VFDs”), because of their high light output, high power light-emitting diodes (“LEDs”), or other bright illumination sources. To offset the demands on the illumination source, HUD implementations may utilize a dielectric or metallic optical coating, which maximizes the amount of light directed toward a viewer's field of vision. However, these optical coatings impart a tinted or colored appearance on optical components located between the viewer and the viewer's field of vision. Attenuation or interference effects of the coatings can produce undesirable appearances. Furthermore, government regulations restrict the degree of attenuation permissible, for example, for an automobile windshield, thereby limiting the extent to which the HUD design can use optical coatings directly mounted on a windshield.
Other HUD implementations rely on Fresnel reflection from a clear optic disposed in the viewer's field of vision, but they require additional mechanical components or wedged combiner optics to mitigate ghost images, or special conditioning of the illumination source to ensure adequate reflection efficiency. Ghost images are caused by multiple reflections from optical boundaries. One HUD implementation for automobiles utilizes slats or louvers embedded in the windshield or optic to be disposed adjacent to the windshield. While this approach extinguishes ghost images and employs a clear optic, the slats obscure the driver's field of vision to some degree. Moreover, the multiple-slat optic is more difficult to manufacture than a single piece optic. Another HUD implementation utilizes a wedged optic that ensures all multiple reflections are optically coincident, thereby superimposing multiple reflections. But, the wedged optic laterally displaces the image. Furthermore, the wedged optic increases the amount of material needed for manufacture of the system, limiting system compactness and increasing system cost.
Conditioning of illumination sources is also needed for existing, optically clear HUD implementations, especially those employing Fresnel reflection and/or dielectric coatings, because illumination sources do not always yield desirable light characteristics at a point of viewing. A specific polarization state from the illumination source is required for many HUD implementations to ensure that an acceptable level of reflection occurs on at least one of the optical boundaries. The physics of Fresnel reflections are such that existing automotive HUD implementations deliver display information with s-polarized light, although p-polarized sunglasses, which are often used by drivers to reduce glare, effectively extinguish s-polarized light beyond visibility. An automotive HUD producing display information in s-polarized light is therefore useless to a driver who is wearing polarized sunglasses.
Thus, current HUD devices suffer from field-of-vision obstruction, display attenuation, interference effects, and ghost images. Manufacturers, designers, and users of HUD devices have therefore recognized the need for a visual display system that minimizes obstruction, attenuation, interference effects, that largely eliminates multiple reflections within a user's field of vision, and that delivers light with desirable polarization characteristics for particular applications.